The present invention is directed to a fiber-optic detector for oils, solvents, and equivalent liquid hydrocarbons, comprising a fiber-optic light transmitter and light receiver, and an optical fiber connected to the same, along with a pick-up for sensing the liquids located at the free end of the optical fiber.
It is possible with this type of apparatus utilizing fiber-optics to detect various kinds of liquids swiftly and safely, especially various oils such as light or heavy fuel oil, or equally other liquids presenting fire or explosive hazards such as solvents. This means are particularly well usable in providing alarm of liquid leakage, because such means have a fairly high speed of detection.
In the chemical industry, dangerous and inflammable liquids, above all, often have to be stored in big cisterns both outdoors and indoors. Because of leakage, such a cistern constitutes a considerable risk to safety of humans as well as nature in general. Electronic detection, particularly of liquids posing risk of fire or explosion, is difficult because an electronic, galvanic measuring and alarm pick-up, involves risk of short-circuiting and sparking. The leakage detector should be "internally safe". This is achievable by using fiber-optics. The fiber is an insulator, and is otherwise completely passive, so that it cannot suffer short-circuiting or cause sparking.
Detection of oils and oil leakage is also rather difficult for the reason that oil is rather passive and, for example, its dielectric constant is little different from that, e.g., of sandy oil. Oil leakage is not observed in general until it is too late. Detection of small leaks may be particularly difficult because it may be hard to notice the leak by the liquid level, e.g. in a large cistern. Moreover, if quick-evaporating liquids are concerned, which are constituted by many kinds of solvents, then the substance may evaporate before the leakage is noticed. The means for detecting leakage should therefore be quite sensitive and fast. Such means provided by the present invention will, for instance, detect acetone within about one second, and fuel oil in less than half a minute. These speeds are adequate.
U.S. Pat. No. 4,386,269 discloses a detector in which a special sheath layer is provided for the fiber, in which the refractive index differs from the refractive index of the fiber itself so that the light is reflected or refracted into the fiber. When such a sheath layer is made oil-absorbing, its refractive index changes so that light will "leak out" and light propagation properties within the fiber change so that it is totally absorbed. It is essential that the oil-absorbing layer is in contact with the core of the fiber in order that the condition for total reflection might be altered. The relationship between the refractive index of the fiber and of its sheath change in a given manner, so that the condition of total reflection is no longer fulfilled for the core and the sheath in immediate contact therewith.
In a detector disclosed in U.S. Pat. No. 4,159,420, a fiber is used which has a refractive index higher than that of the surrounding matter. The fiber is surrounded with a material of which the refractive index changes when it comes into contact, e.g., with oil. This change of refractive index has the effect that the condition of total reflection in the core of the fiber is no longer fulfilled and the light "leaks out", thus causing considerable extra attenuation in the propagation of light. On the end of the fiber, a mirror may be installed, the reflection from the mirror returning the light to its starting point, whereby the change of refractive index can be observed when the light is reflected by the walls of the fiber. It is essential that the relationship of the refractive indices of the fiber core and of its sheath change in a given manner, so that the condition of total reflection is no longer fulfilled for the core and for the sheath in immediate contact therewith.
In E.P. Application No. 126,600, optical fibers are employed to observe the presence of certain chemicals by using on an end of the pick-up, a pick-up construction of such kind wherein the end of the fiber bundle is tightly surrounded by a porous membrane inside which powdery, cross-linked polystyrene has been provided. To such polystyrene powder, a reagent has been chemically bound which changes color on contact with the liquid to be monitored. The porous membrane consists of Teflon, which repels water. The chemical to be monitored passes through the Teflon film and reacts with the substance provided in the polystyrene powder, the spectroscopic characteristics of the substance changing on contact with the liquid.
In reference WO 86/05589, a material is used to which an indicator substance has been permanently attached of which the spectral characteristics change when it comes into contact with the substance to be monitored.
G.B. Pat. No. 21 03 786 discloses a detector in which the surface of the fiber core has been covered with an indicator substance having a porous surface. The end of the optical fiber is provided with a mirror which reflects the light back to the measuring electronics.
It may be noted regarding the prevailing state of the art, that it is previously known that optical fibers may be used to observe sensor elements installed on the end of the fiber. Light is sent from an optical fiber transmitter by the fiber to the object to be examined, and the light is returned back to a detector along the same, or another fiber. The sensor element on the end of the fiber is then affected in one way or another by the phenomenon that is being examined.
A leakage detector is known in the art which detects the presence of oils based on the circumstance that the sheath material of the optical fiber consists of a material which swells as it comes onto contact with oil. The oil then gains access to the space between the sheath and fiber, thus causing extra attenuation in the propogation of light.